Recently, the range of use of a secondary battery is remarkably increasing. For example, the secondary battery is used as power source of an electrical appliance, e.g., a digital camera, a digital video camera, and a notebook computer; a communication device, e.g., a cellular phone; a power apparatus, e.g., an electric power tool and a vacuum cleaner; or the like. The term “the secondary battery” means a rechargeable battery that can repeat charge and discharge cycles, and during charge, electrical energy is transformed into chemical energy stored in the secondary battery. The stored chemical energy is converted back into electrical energy so as to be used. A nickel-cadmium battery, a nickel metal hydride battery, a lithium ion battery, a NAS battery, etc. are listed as practically used secondary batteries.
Incidentally, the electromotive reaction and the discharge reaction inside the secondary battery include chemical reaction, electric reaction, and complicated energy conversion and energy transfer where the chemical reaction and the electric reaction are mutually concerned. There lies time element among these various reactions. Accordingly, the secondary battery must be charged in consideration of the reactions, and when excessive current flows through the secondary battery in the process of charge, the internal structure of the secondary battery is sometimes damaged by irreversible chemical reactions, including unexpected exothermic reaction, swelling reaction, or the like. Even if the internal structure of the secondary battery is not damaged, it is degraded so that the battery life becomes shorter, and effective cycle of the battery is decreased.
Conventionally, as disclosed in many patent applications, to charge the secondary battery appropriately, a program for changing voltage to the secondary battery with the passage of charging time is programmed into a charge control device of a charging equipment for the secondary battery, so that the voltage controlled according to the program is applied to the secondary battery. The end of charge of a battery is judged and controlled on the basis of detection of the battery voltage as a variable controlled by a charging equipment, where a voltage detection device for detecting voltage of the secondary battery is provided.
For example, the charging equipment for a secondary battery disclosed in the Japanese Patent Laid Open Gazette Hei. 8-9563 comprises a voltage detection circuit for detecting the minus potential difference of charging voltage applied to a battery with constant current, a temperature detection circuit for detecting change of the battery temperature per unit time (temperature differential value) generated in the charged battery with constant current, and a charge control circuit for controlling an on-off switching for charging basing on comparing the minus potential difference detected by the voltage detection circuit and the temperature differential value detected by the temperature detection circuit with their respective preset standard values. If the minus potential difference and the temperature differential value reach their preset standard values thereof respectively, charging of the battery is halted. In this way, there is such well-known conventional charging control device which observes the charged condition of a battery on the basis of detection the battery voltage or the temperature serving as a controlled variable and judges whether charge of the charging battery should be finished or not.
However, when the method for deciding end of charging basing the above-mentioned variable is simply applied in disregard of the charged condition of the secondary battery, various problems arise as follows: The characteristic property of the secondary battery during its charging varies according to variation of an electrode, an electrolyte, a structure, or another element of the charged secondary battery. Even if the secondary batteries are of the same kind or marked with the same type code, the characteristic property thereof varies with the difference of an environmental condition during charge, a use history, an electrochemical itinerancy in the secondary battery, and the like. Therefore, a battery charged according to the conventional control method may be overcharged, where the problem arises that abnormal chemical reactions (irreversible chemical reactions) occurs inside the secondary battery so as to generate heat, that is, electrical energy is transformed into heat energy, whereby the charging efficiency may be decreased, and gas may be generated in the secondary battery so as to increase the internal pressure thereof and cause the leak of liquid from the secondary battery. Therefore, the solid internal structure of the secondary battery required for repeating charge and discharge cycles is so damaged as to decrease effective battery cycles of the secondary battery.
Moreover, it is desirable that the charging time of the secondary battery is as short as possible. However, as mentioned above, the conventional charging control device using the fixed charge control pattern applies the same voltage to any type of secondary. Therefore, if a secondary battery is charged by voltage lower than its rated voltage, it takes quite a long time to fully charge the secondary battery. In addition, the conventional device inconveniently has no means for informing a user how much electricity the secondary battery to be charged stores, and how long it takes to charge the secondary battery.